feat: add support for auto-pairs

[altacountbabi]

Adds InsertPair and BackspacePair edit commands

[kronberger-droid]

@altacountbabi
Love it, great idea thanks.

We are close to a code freeze before the next release tho, so I think it makes sense to wait until after to merge.
@fdncred or do you think its worth propagating it into Nushell before the release?

[fdncred]

@kronberger-droid I'd probably wait on this one until after the release. I'd also like to see a nushell counterpart PR to this one that enables or disables this feature.

[eitsupi]

I've independently implemented auto-pairs in a reedline-based project (https://github.com/eitsupi/arf), so I'd like to share some issues I encountered.

Two known pitfalls with this design

History selection leaves orphan closing chars

When the cursor is inside a pair (e.g. foo(|)) and the user selects a history entry, reedline's replace_in_buffer uses span.end = cursor_pos, so it replaces only foo( with foo(), leaving the original ) behind → foo()|). The engine would need a post-replacement cleanup step when auto-pairs is active.

Paste duplicates closing chars

Pasting () produces ()). This requires use_bracketed_paste(true) to avoid; the example and docs should call this out explicitly (or with_auto_pairs() could enable it implicitly).

Design suggestion

The AutoPairs struct has no way to skip insertion based on context, e.g., don't insert inside a string literal. A predicate like:

pub fn with_skip_if<F: Fn(&str, usize, char) -> bool + Send + Sync + 'static>(mut self, f: F) -> Self

would let callers inject language-specific logic without shadow state.
Without it, projects with quote-awareness will still need to work around this at the keybinding layer.

[kronberger-droid]

@fdncred
I am really tempted, since I think it was definitely a missing piece. But I don't have enough time until the release to test out the blast radius, so lets wait.

But @eitsupi comment makes waiting even more reasonable.

[fdncred]

But @eitsupi comment makes waiting even more reasonable.

Ya, I'd like to see some code to help with this. I've experienced this in code editors and it's a pain sometimes, "NO, I ALREADY HAD A CLOSING PAREN. DON'T ADD ANOTHER!!!!"

I probably wouldn't want to land this without something helping with it aggressively closing pairs.

[kronberger-droid]

"NO, I ALREADY HAD A CLOSING PAREN. DON'T ADD ANOTHER!!!!"

Haha very relatable.

Yeah let's try to get this right.
@eitsupi every input is welcome!